If the power tool is a plunge-cut circular saw its circular saw blade is shielded by a guard, except for when the housing is pushed towards the foot plate so that the saw blade protrudes through an aperture in the foot plate to perform a cutting operation. The housing and saw blade of a plunge-cut circular saw move relative to the foot plate about a pivot connecting the housing and foot plate.
If the power tool is a router it normally does not have a guard because its router bit is at least partially shielded by a pair of support columns, but the router blade does protrude through an aperture in the foot plate to perform a cutting operation when the housing is pushed towards the foot plate. The housing and router blade move relative to the foot plate in a straight line path guided by the pair of support columns which connect the housing and foot plate.
At the point of maximum protrusion of the circular saw blade, or the router bit, as the case may be, through the footplate, the housing is said to be in the “plunged” position.
An interlock may be used to control the extent to which the housing can move towards the foot plate and the conditions under which such movement occurs, like, for example, the isolation or de-isolation of the on-off switch, or locking or unlocking of the housing's movement or the blade's rotation.
FIG. 3 of patent publication DE 40 23 101 discloses an embodiment of a plunge saw comprising a tool housing displaceably mounted to a foot plate, a rotatable blade the rotational axis of which is fixed in relation to the housing, an electric motor for driving the blade, an electric switch manually operable by a switch button to energize the motor; and an interlock for controlling movement of the housing with respect to the foot plate. The interlock has a manually operable control button mounted to the housing and the control button cooperates with a side of a rail which is fixed in relation to the foot plate. This enables the control button to select between a plunge cut mode whereby movement of the housing is limited to between a parked position whereat the blade is entirely above the foot plate and a plunged position whereat the blade protrudes below the foot plate, or a blade change mode whereby movement of the housing is limited to between the parked position and a “blade change” position whereat the blade is prepared for a blade change operation.
When blade change mode is selected, the control button maintains isolation of the on-off switch by blocking manual movement of the switch button. This prevents energization of the motor. The housing is moveable into the blade change position. However, if ever the housing is accidentally released it naturally returns to the parked position under the bias of a plunge spring. The control button adopts a default mode which prevents the housing from leaving the parked position. The user must take care to maintain pressure on the housing otherwise blade change mode is automatically de-selected. It can be re-selected, but this requires repetition of a two-stage process which may require two hands: one hand to operate the control button and another hand to push the housing toward the foot plate. No hand is free to steady the plunge saw.
When plunge cut mode is selected, the control button de-isolates the on-off switch by unblocking movement of the switch button. Subsequent manual movement of the switch button energizes the motor and latches the switch button behind the control button to hold the latter in situ. The housing can move between the parked and plunge cut positions. However, if ever the switch button is accidentally released it naturally de-energizes the motor and unlatches the control button. The blade stops rotating and, if the control button is not manually held in situ, the control button adopts either the blade change mode, or, if the housing hasn't yet descended towards the plunged position, a default mode which prevents from leaving the parked position. Again, the user must take care to maintain pressure on the switch button otherwise plunge cut mode is automatically de-selected. It can be re-selected, but, again, this requires a two-stage process which may require two hands: one hand to operate the control button and another hand to operate the switch button. No hand is free to steady the plunge saw or a work piece upon which the plunge saw is performing a cutting operation.